(a) Field of the Invention
The present invention relates to a display device for displaying images, and more particularly to a shift register for driving an image display device and a display device including the same.
(b) Description of Related Art
Recently, a wide variety of flat panel displays have been developed such as organic electroluminescence displays (OLEDs), plasma display panels (PDPs) and liquid crystal displays (LCDs) instead of heavy and large cathode ray tubes (CRTs).
The PDPs are devices which display characters or images using plasma generated by a gas-discharge. The OLEDs are devices which display characters or images by applying an electric field to specific light-emitting organics or high molecule materials. The LCDs are devices which display images by applying an electric field to a liquid crystal layer disposed between two panels and regulating the strength of the electric field to adjust a transmittance of light passing through the liquid crystal layer.
Of these flat panel displays, for example, the LCDs and the OLEDs each include a panel unit provided with pixels including switching elements and display signal lines, and a gate driver, i.e., a shift register providing a gate signal for gate lines of the display signal lines to turn on/off the switching elements. The shift register is comprised of a plurality of stages. Each of the stages includes a plurality of NMOS or PMOS transistors comprising an input unit, an output unit, a pull-up driving unit and pull-down driving unit.
A first stage outputs the gate signal synchronized with a clock signal in response to a vertical synchronization start signal, and remaining stages, including a second stage, output the gate signal synchronized with the clock signal in response to gate outputs of previous and next stages.
The shift register described above has a reliability problem during low temperature driving. During low temperature driving at about 20 degrees below zero, drain currents of transistors of each stage decrease due to characteristics of semiconductor material used to make the transistors. Thus, retardation occurs, which is a phenomenon characterized by an increase in time taken for an output level of the transistors of the output unit to rise to a desired level. Additionally, resistive-capacitive (RC) delay due to resistances and parasitic capacitances of the gate lines adds to the impact of retardation. Such impact of retardation can affect next stages causing a gate block defect of not turning on the switching elements of a portion of the pixels.